1. Field of the Invention
This invention relates to building materials and methods for making the same, and more particularly to the addition of low density additives (LDA) into cementitious cellulose fiber-reinforced building materials.
2. Description of the Related Art
Fiber-reinforced cement (FRC) products such as water-resistant building sheets have been used for building since 1895. In recent history reinforcing fibers used in such products have included not only asbestos fibers, but also cellulose fibers (see Australian Patent No. 515151), metal fibers, glass fibers and other natural and synthetic fibers. Typically, the density of such building sheets is from about 1.2-1.7 g/cm3, the variation in density typically being achievable by compression and dewatering of the fiber cement slurries used in manufacture and by varying the amount of fiber used. At these densities, the cement based matrix has few voids, which results in lower water absorption which has usually been considered necessary for good durability performance of cement matrices.
The densities of fiber cement described above mean the products are heavier than timber based products of equal dimension and have reduced workability. Workability encompasses the ease with which a board is handled and installed. Therefore, fiber cement building products are more difficult to cut, machine and nail than timber and timber based products. In this regard, the density of natural timber sheets typically ranges from about 0.7-0.9 g/cm3 for dry hardwoods and from about 0.38-0.6 g/cm3 for dry softwoods. Thus, a density-modified fiber cement material with density similar to timber may be expected to improve workability and enable lighter, more nailable, easier to cut and easier to machine products to be manufactured. However, this would have to be achieved while retaining the durability, fire resistant, rot proof and water resistant properties of fiber cement if the density modified fiber cement is to be used in the same range of applications.
Prior art describes how lightweight inorganic powders can be added as density modifiers in cement or fiber-reinforced cement materials. Low density additives for FRC products are defined as having a loose bulk density of about 0.8 g/cm3 (about 50 lbs./cu.ft.) or less. The typical low density additives (LDA) used include low bulk density calcium silicate hydrates (CSH), expanded polystyrene beads (EPS), expanded vermiculite, expanded perlite, expanded shale, and expanded clay. The density modification of cement-based materials with such inorganic particles is primarily achieved by introducing porosity into the material. Typically, the pore spaces are filled with water when the material is submerged in water or exposed to rain for a length of time. This causes these materials to have poorer wet to dry dimensional stability (moisture resistance), a higher saturated mass, and poor freeze-thaw resistance.
Accordingly, there is a need for a lightweight FRC building material and method for manufacturing the same with improved wet to dry dimensional stability over that of typical density modified products. Secondly, the lightweight building material should maintain similar wet to dry dimensional stability as that of FRC products without density modifiers if the density modified material is to be used in the same range of applications. In addition, it is highly preferred in some applications that the material also have a low saturated mass, good freeze-thaw resistance, and high temperature dimensional stability. Finally, it is also desirable to have a FRC building product where lower ranges of densities closer to that of timber and timber based products can be achieved with improved durability.
Two low density additives have been evaluated that have properties more desirable to FRC building materials than typical low density additives. These two low density additives are volcanic ash and hollow ceramic microspheres. One embodiment of the invention includes the addition of volcanic ash (VA) into an FRC building material. A second embodiment comprises the addition of hollow ceramic microspheres (microspheres) into the FRC building material. A third embodiment incorporates the blending of microspheres with volcanic ash and/or other typical low density additives into the FRC building material. The third embodiment with the blend of microspheres and VA and/or other low density additives may be more preferable than the first embodiment with the introduction of volcanic ash by itself. The second embodiment with the addition of microspheres by themselves may be more preferable than either the first or third embodiments as described above, depending on the properties being considered for a particular application.
Compared to current FRC products, one advantage of the first embodiment with volcanic ash is that it provides the product with low densities and improved workability at an economical price, as well as improved dimensional stability over that of typical low density additives.
The second embodiment encompasses the addition of microspheres in fiber-cement products. Compared to current FRC products, the benefits of adding microspheres include the low density and improved workability of the product without increased moisture expansion or freeze-thaw degradation associated with the addition of lightweight inorganic materials to FRC mixes. Moreover, the addition of microspheres provides improved thermal dimensional stability for FRC material.
The third embodiment relates to the addition of microspheres in combination with VA and/or other typical low density additives in FRC material. Blending microspheres with other low density additives is advantageous because lower density FRC products can be achieved with less weight percent addition (as compared to microspheres only) due to the lower densities of VA and other typical LDA relative to microspheres. This also enables fiber cement products to achieve lower density ranges to further improve workability, while microspheres minimize the adverse effects typical low density additives have on wet-to-dry dimensional stability and overall durability.
Thus, in one aspect of the present invention, a building material is provided comprising a fiber-reinforced cement formulation and a low density additive incorporated into the formulation. The addition of the low density additive to the formulation lowers the density of the building material as compared to a building material having an equivalent fiber-reinforced cement formulation without the low density additive, while at the same time the building material with the low density additive has less than about a 20% increase in moisture expansion as compared to a building material having an equivalent fiber-reinforced cement formulation without the low density additive. More preferably, the addition of the low density additive to the formulation lowers the density of the building material as compared to a building material having an equivalent fiber-reinforced cement formulation without the low density additive, while at the same time the low density additive either maintains or decreases the moisture expansion of the building material as compared to a building material having an equivalent fiber-reinforced cement formulation without the low density additive. The density of the building material is preferably about 1.2 g/cm3 or less.
In another aspect of the present invention, a building material formulation is provided to form a building product. The formulation comprises a hydraulic binder, an aggregate, fibers and volcanic ash. In one embodiment, the volcanic ash improves the workability and lowers the density of the final building product by more than about 10% as compared to a building product made from an equivalent formulation without volcanic ash. In another embodiment, the formulation with volcanic ash has a negligible difference in the moisture expansion of the final product whereby the product either maintains or increases moisture expansion by less than about 20% as compared to a building product made from an equivalent formulation without volcanic ash. For the degree of density modification achieved, this moisture movement increase is surprisingly low. With nominally the same formulation ingredients, it has been found that differences in moisture expansion for volcanic ash formulations exist. Such differences are primarily due to fluctuations in the surface area of raw materials.
In another aspect of the present invention, a method of forming a low density building material is provided. Hydraulic binder, aggregate, volcanic ash and water are mixed to create a slurry. The slurry is processed into a green shaped article. The green shaped article is cured to form the low density building material. In one embodiment, the article is cured by autoclaving. In another embodiment, the low density building material formed has a density of about 1.2 g/cm3 or less, and a moisture expansion of about 0.17% or less.
In another aspect of the present invention, a building material formulation comprises a hydraulic binder, an aggregate, fibers and hollow ceramic microspheres. The final building material has a density of about 1.2 g/cm3 or less. In one embodiment, about 4.1%-15% cellulose fibers are provided in the formulation. In one preferred embodiment, the microspheres lower the density of the final building product by more than about 15%, even more preferably more than about 30%, as compared to a building product made from an equivalent formulation without microspheres. In another embodiment, the microspheres decrease the moisture expansion of the final product as compared to a building product made from an equivalent formulation without microspheres, preferably by more than about 5%, more preferably by more than about 10%. In one preferred embodiment, a combination of microspheres with other additional low density additives such as volcanic ash and/or low bulk density CSH are provided in the formulation.
In another aspect of the present invention, a method of forming a low density building material, comprising mixing hydraulic binder, aggregate, fibers, hollow ceramic microspheres and water to create a slurry. The slurry is processed into a green shaped article. The green shaped article is cured to form the low density building material. The resulting building material has a density of about 1.2 g/cm3 or less. In one embodiment, more than about 4% fibers are mixed to create the slurry. In another embodiment, the article is cured by autoclaving.